Reverse a Linked List

The Idea

Walk the list once, turning each link to face the node behind it. Three pointers do the work: prev trails, node is current, and a temporary holds the rest of the chain before you overwrite it. The old tail becomes the new head.

Real-World Example

A bucket brigade reversing direction after the fire is out. Nobody changes places; each person simply turns to face the neighbour they used to receive from. The last person in the chain is suddenly the one who starts every pass.

The Code

class Node:
    def __init__(self, v): self.value, self.next = v, None

def reverse(head):
    prev, node = None, head
    while node:
        nxt = node.next        # remember the rest of the list
        node.next = prev       # flip this link backwards
        prev, node = node, nxt # slide both pointers forward
    return prev                # the old tail is the new head

head = Node(1); head.next = Node(2); head.next.next = Node(3)
node = reverse(head)
while node:                    # prints 3 2 1
    print(node.value, end=" "); node = node.next

class Node:
  def __init__(self, v): self.value, self.next = v, None

a = Node(1); a.next = Node(2)
prev = None
while a:
  nxt = a.next
  a.next = prev
  prev, a = a, nxt
print(prev.value, prev.next.value)